This invention relates to a charged particle beam device for the examination of specimens. In particular, this invention relates to the beam column used for guiding the charged particle beam.
Beams of negatively or positively charged particles can be used for the examination of specimens. Compared to optical light, the resolving power of a beam of charged particles is several magnitudes higher and allows for the examination of much finer details. In charged particle beam devices, electric or magnetic fields, or a combination thereof, act upon the beam in a manner analogous to that in which an optical lens acts upon a light beam. In particular, any electric or magnetic field which is symmetrical about an axis is capable of forming either a real or a virtual charged particle image. Hence, an axially symmetric electric or magnetic field is analogous to a spherical lens. Furthermore, similar to light-optics, apertures are also used in charged particle devices. The primary use of these apertures is to limit the diameter of the beam of charged particles or to eliminate stray or widely scattered particles. However, in charged particle devices, apertures are an easy target for contamination caused by hydrocarbons.
Charged particles on their path from particle source to the specimen to be examined are strongly scattered by all forms of matter including air. Hence, the entire instrument must, in general, be evacuated. Nevertheless, the presence of hydrocarbon molecules in vacuum chambers is virtually unavoidable. These are commonly formed in vacuum chambers of the charged particle devices as a result of hydrocarbons and silicon oils migrating from vacuum pumps or evaporated from vacuum seals. Radiation of hydrocarbon molecules with charged particles leads to cracking of bonds and to the creation of carbon double bonds. The formation of carbon double bonds, in turn, results in cross linking and the final product will be a carbonaceous polymerized substance. In particular, the edges of apertures which serve to limit the diameter of a particle beam are exposed to particle radiation. At these edges, carbon-rich films or contamination needles easily form and grow into the openings which: change the shape of the passing beam.
Furthermore, these contaminations protruding into the openings are getting charged by the particle beam. The impinging particles are absorbed by the protrusions which could primarily be classified as insulators. The charge build up causes the passing particle beam to deflect and results in imaging artifacts. Due to the constant accumulation of charge, the voltage of a contamination increases until it reaches the break down point. At that point, a sudden discharge will take place and the imaging artifact, caused by charging, disappears. The subsequent absorption of charged particles will again build up the voltage of the contamination until it reaches the break down point. This cyclical process results in a periodic artifact of image flickering. Additionally, there is the artifact caused by the slowly growing contamination layer at the edge of an aperture which steadily narrows the diameter of the passing beam.
In some devices multi apertures have been used for obtaining a variety of preselected beam diameters. A plate comprising several apertures with distinct diameters is placed between particle source and specimen. The beam of charged particles is then guided through one of these apertures in order to reduce it to a desired diameter before it impinges onto the specimen to be examined. Without limiting the scope of the invention, the following explanations will primarily concentrate on the use of electrons as charged particles. An impinging beam of electrons with a given electron density and a bigger beam diameter causes more primary electrons to hit the target. The higher number of interactions between primary electrons and target result, in general, in an increase of secondary products being detected and, consequently, in a higher imaging contrast. On the other hand, a smaller beam diameter,with fewer primary electrons getting absorbed by the target causes a lesser amount of charging and allows for focusing the beam to a smaller diameter in the sample plane.
In particular small apertures of multi aperture units require frequent cleaning due to high intensity radiation of their edges. Furthermore, frequent cleaning is necessary because of the large influence a contamination spot of given size has with respect to the total surface of a small apertures. For cleaning, the part of the electron beam column containing the multi aperture unit needs to be opened and its vacuum broken. After cleaning, time consuming realignment and adjustment steps are necessary before the electron beam device is fully operational again. Since this procedure results in considerable machine down-time, it is desirable to increase the interval at which such apertures need to be cleaned.
In the past, a variety of attempts have been made to reduce contamination of apertures e.g. use of hydrocarbon free vacuum and appropriate prior cleaning of vacuum chamber and aperture unit. In another attempt, apertures were heated during machine running time. The increased Brownian movement of the molecules at the edges of the apertures prevents the formation of contamination layers thereon. Yet, the installation of heaters in the vicinity of apertures is burdensome and costly.
The present invention intends to provide an improved charged beam column for examining a specimen with a charged particle beam. According to one aspect of the present invention, there is provided an apparatus as specified in claim 1.
According to a further aspect, the present invention also provides a method as specified in claim 11.
Further advantageous features, aspects and details of the invention are evident from the dependent claims, the description and the accompanying drawings. The claims are intended to be understood as a first non-limiting approach to define the invention in general terms.
According to preferred aspect there is provided a charged particle beam column with a first vacuum chamber. The charged particle beam device further comprises a particle source for providing a beam of charged particles and a multi aperture unit with at least two beam defining apertures for shaping the beam of charged particles. The particle source and the beam defining apertures are located within the first vacuum chamber. A separation unit for isolating a second vacuum chamber from the first vacuum chamber whereby the separation unit comprises a path aperture for the charged particle beam is arranged between the first and second vacuum chamber. A first deflecting unit directs the beam of charged particles through one of the beam defining apertures and a second deflecting unit directs the beam of charged particles through the path aperture.
According to a preferred aspect of the present invention, the first and or second deflecting unit is located outside the first vacuum chamber. This allows the use of smaller vacuum chambers and disposes of the need to provide sealing members for cables and connectors used to operate the deflecting units.
In a further preferred aspect of the present invention, the second deflecting unit guiding the beam of charged particles through the path aperture comprises two stages. The first one guides the beam coming through a selected beam defining aperture back to the optical axis. Then, the second stage guides it along the optical axis or, alternatively, in close vicinity to the optical axis. Advantageously, this allows the beam to be directed onto a trajectory close to the optical axis even before it passes through the path aperture of the separation unit. The second deflection unit already guides the beam to a direction so that it propagates towards the target or specimen without having to be directed again by a third deflecting unit.
According to a still further aspect of the present invention, there is provided a third deflection unit for directing the beam of charged particles through the objective lens after it has passed through the path aperture of the separation unit. In some preferred embodiments, the trajectory of the beam is tilted with respect to the optical axis after it has passed the first and second deflection unit. The third deflection unit, advantageously, compensates excessive tilt angles and redirects the charged particle beam so that the angle between the optical axis and the beam is reduced. Preferably, the third deflection unit is located in between the separation unit and the specimen.
In a preferred embodiment according to the invention, the third deflection unit comprises two deflection stages. The first stage of the deflection unit redirects the beam towards the optical axis and, subsequently, the second stage guides the beam along the optical axis or in close vicinity thereto. This preferred embodiment not only allows for a reduction of the angle between optical axis and beam, but also allows to the beam to propagate along, or in close vicinity, to the optical axis. Furthermore, in case the optical axes defined by path aperture and objective lens are not coaxial, the second stage allows the beam to be redirected along the optical axis as defined by the objective lens.
In another preferred embodiment, the vacuum in the first vacuum chamber is higher than the vacuum in the second chamber. Advantageously, the multi aperture located in the first vacuum chamber is kept at a higher vacuum. Thus, the time span in which contaminants develop at the path aperture and start to negatively influence the trajectory of the charged particle beam is slowed down by specifically reducing the number of hydrocarbons in this area. At the same time it is not necessary to maintain the vacuum level of the first vacuum chamber in parts of the beam column where contaminants have lesser disturbing influence.
In still another preferred embodiment, the first vacuum chamber is kept at ultra high vacuum. At this vacuum level, the number of hydrocarbons in the vicinity of the path defining apertures are drastically reduced which increases machine running times.
The invention is also directed to methods by which the described apparatus operates. It includes method steps for carrying out every function of the apparatus. These method steps may be performed by way of hardware components, a computer programmed by appropriate software, by any combination of the two or in any other manner.